1 of 20

Magnetic disk and tape – Recorders, digital plotters and printers, CRT display, DSO, Lissajous pattern LED, LCD & Dot matrix display – Data Loggers - Software implementation of scopes for different input

​

UNIT IV STORAGE AND DISPLAY DEVICES

​

https://eeeviews.blogspot.com/

2 of 20

The LEDs or their display require_____________

  1. A voltage of 1.2V and a current of 20mA
  2. A voltage of 25 V and a current of 20mA
  3. A voltage of 1.2V and a current of 100mA
  4. A voltage of 25 V and a current of 100mA

​

​

​

​

REVIEW

​

https://eeeviews.blogspot.com/

3 of 20

The switching time of LEDs is of the order of_____________

  1. 1s
  2. 1ms
  3. 1ns
  4. All the above

​

​

​

​

REVIEW

​

https://eeeviews.blogspot.com/

4 of 20

LEDs emit light only in _____

  1. Red colour
  2. Yellow colour
  3. Green colour
  4. Red, green, yellow and amber colour

​

REVIEW

​

https://eeeviews.blogspot.com/

5 of 20

An LCD requires a power of ___ to light up one segment of a seven segmented display system.

  1. 20W
  2. 20mW
  3. 20μW
  4. 20nW

​

​

​

REVIEW

​

https://eeeviews.blogspot.com/

6 of 20

The switching time of LCD is of the order of_____________

  1. 1s
  2. 10ms
  3. 10ns
  4. All the above

​

​

​

​

REVIEW

​

https://eeeviews.blogspot.com/

7 of 20

  • Luminous spot is produced by a beam of electrons striking a fluorescent screen.
  • Measure signal of about 20GHz.
  • Only detect a repetitive waveform
  • DSO

CRO

​

https://eeeviews.blogspot.com/

8 of 20

​

CRT

​

https://eeeviews.blogspot.com/

  • Electron gun assembly
  • Deflection plate assembly
  • Fluorescent screen
  • Glass envelope
  • Base, through which connections are made to various parts.

9 of 20

  • Barium and strontium oxide deposited on the cathode to obtain high emission of electrons.
  • Control grid is usually a nickel cylinder, with a centrally located hole.
  • Pre accelerating anode and the accelerating anode are connected to a positive high voltage of about 1500V.
  • Types of focusing
    • Electrostatic focusing
    • Electromagnetic focusing

CRT- Electron gun

​

https://eeeviews.blogspot.com/

10 of 20

  • One pair of plates is mounted horizontally and produce an electric field in the vertical plane. This pair produces a vertical deflection and is thus called Vertical deflection plates or Y plates.
  • Other pair of plates is mounted vertically and produce horizontal deflection or X plates.

CRT- Deflection plates

​

https://eeeviews.blogspot.com/

11 of 20

  • The front of the CRT is called the face plate(100x100mm).
  • The inside surface of the face plate is coated with phosphor. The electron beam on striking the phosphor gives off light and heat.
  • A thin film of metal, such as aluminium(0.1-0.25μm) is usually deposited on the non viewing side
    • Metal layer act as a heat sink and reduce th danger of phosphor burn.
    • Light scatter from the phosphor is reduced, and the aluminium reflects light going back into the tube towards the viewer, so increasing the brightness
    • Electron which strike phosphor release secondary electrons. These secondary electrons are usually collected by the aquadag. Under certain conditions the electron may retain on the screen, and cause a negative voltage build up. This would decrease the accelerating voltage of the beam. This charge build up is prevented by the aluminium layer, which is connected electrically to the aquadag.

Screen for CRT

​

https://eeeviews.blogspot.com/

12 of 20

Graticule - CRO

​

https://eeeviews.blogspot.com/

  • Graticule (8x10 pattern) is a grid of lines that serves as a scale when making time and amplitude measurements.

​

13 of 20

Simplified block diagram of a CR0

​

https://eeeviews.blogspot.com/

14 of 20

  • Retaining the image on the screen for longer.
  • Analog /Digital

Storage Oscilloscopes

https://eeeviews.blogspot.com/

15 of 20

Digital Storage Oscilloscopes

https://eeeviews.blogspot.com/

16 of 20

Digital Storage Oscilloscopes

https://eeeviews.blogspot.com/

Digitising occurs by taking a sample of the input waveform at periodic intervals.

​

Sampling rate must be at least twice as fast as the high frequency in the input signal. If this is not done then aliasing will result.

​

High sampling rate means that the digitiser, which is an analog to digital converters, must have a fast conversion rate. This usually requires expensive flash analog to digital converter.

17 of 20

Digital Storage Oscilloscopes

https://eeeviews.blogspot.com/

One method of overcoming the need for a high performance converter is to use an analog store as in fig.

This method allows operation at upto 100 mega samples per second.

Advantage: Low cost

Disadvantage: cannot accept data during digitising period

18 of 20

Digital Storage Oscilloscopes

https://eeeviews.blogspot.com/

Analog storage oscilloscope

Digital storage oscilloscope

Higher bandwidth and writing speed

Limited in speed

CRT is costlier

CRT is much cheaper than an analog storage oscilloscope

Low resolution

High resolution

19 of 20

Liquid Crystal Display (LCD)

https://eeeviews.blogspot.com/

  • Dynamic scattering
  • Field effect type

Advantages: Low power consumption & Low cost

Disadvantages: Slow devices &occupies large area

20 of 20

Light Emitting Diode(LED)

https://eeeviews.blogspot.com/

  • Gallium arsenide phosphide – red or yellow
  • Gallium arsenide – green or red

Advantages: switching time is less than 1ns & miniature in size

Disadvantages: High cost (large area display)